The present invention relates to a multi-layer paper machine screen, for example to a sheet forming screen of a paper machine, especially to a sheet forming screen as it is used in the process of papermaking in the sheet forming section of a wet end of a paper machine for draining/filtration of a fiber suspension and of a paper fibrous material, respectively. Screens of this type are mainly used for high-quality graphic types of paper and packaging papers having a low paper weight/grammage and high requirements concerning printability. These papers can be produced with so-called gap formers at speeds of more than 2,000 m/min. In this respect, high demands are made on the screen's mechanical stability, draining performance, fiber support, reduced tendency to marking and lifespan.
An essential process in papermaking is the forming of the sheet (=sheet forming) which is effected by draining/dewatering a fiber suspension or paper fibrous material by means of filtration in the sheet forming section of the wet end of a paper machine by using a so-called sheet forming screen.
The fiber suspension may be understood as a mixture of mechanical pulp fibers or chemical pulp fibers, fillers and auxiliary chemical agents suspended in water.
In order to be able to produce a paper sheet as uniform as possible, it is necessary to increase or set the amount of water within the fiber suspension immediately before the sheet formation to approximately 99%. This ensures that the fibers can be distributed uniformly in the water, which is beneficial to the quality of the sheet to be formed.
The amount of water is reduced to approximately 80% by the above-mentioned filtration process within the sheet forming section, i.e. during the sheet forming process. The paper fibers and the fillers and auxiliary agents remain on the paper machine screen in a uniformly distributed manner in the form of a nonwoven fabric.
While in the past the draining process took place mainly by means of a paper machine screen applied to a Fourdrinier paper machine/long screen machine, mainly twin screening machines are being used these days, for example in the form of so-called gap formers. These twin screening machines are characterized in that the fiber suspension is injected into a gap which is formed between two paper machine screens, so that draining can take place simultaneously through both screens, whereby it is possible to significantly accelerate the filtration process and thus also the production rate of the papermaking machine. There are papermaking machines these days for types of paper having a low grammage, which are capable of producing with speeds of more than 2,000 m/min.
The extreme requirements for the paper to be produced and the conditions existing in the papermaking machine require specifically designed sheet forming screens which offer/comprise high fiber support, high openness and a high mechanical stability at the same time. In addition, a low tendency to marking of the fabric is necessary in particular for the domain of graphic papers.
Multi-layer paper machine screens have proven of value for these fields of application over the past years, comprising two sides formed in a different manner, which are adapted to the respective purpose of use. Screens of this type have a paper side which is formed by the upper side of an upper fabric. In habitual language use, the paper side is also referred to as the upper side of the screen, and is relevant for forming the paper sheet. In addition, these screens have a machine side which is formed by the lower side of a lower fabric. The machine side which can also be referred to as the lower side of the screen contacts the members of the papermaking machine. The respective screen side has a machine direction (generally the longitudinal direction) and a cross direction; in this respect, machine direction (also MD for “machine direction”) refers to the running direction of the paper web and therefore also to the running direction of the paper machine screen, and the cross direction (also CMD for “cross machine direction”), sometimes also referred to as cross machine direction, is the direction turned by 90° in the plane of the paper machine screen, i.e. the direction located transverse to the running direction of the paper and the screen.
Due to the very specific configuration of modern paper machine screens, usually neither the paper and machine side nor the longitudinal/machine and cross direction are interchangeable, as otherwise the mode of operation of the screen would not be ensured or would not be ensured sufficiently. For example, the machine direction threads (=longitudinal threads) on the machine side which realize circulation of the screen, can be protected against wear by transverse threads projecting or protruding significantly. For example, providing a balanced ratio of longitudinal and transverse threads on the paper side can ensure a good depositing possibility for the paper fibers. With respect to the fiber support, but also with respect to the tendency to marking of the screen, the most simple and at the same time the oldest basic weave of textile engineering has proven of value for the upper fabric and thus for the paper side, namely the so-called plain weave. In this kind of weave, the repeat (=the smallest repeating unit of the weave) of which is formed exactly by two warp threads (as a general rule, the longitudinal threads/machine direction threads of the screen are formed by the warp threads) and two weft threads (as a general rule, the transverse threads of the screen are formed by the weft threads), the threads are connected to a fabric in a particularly close and uniform manner. Although the plain weave is very well suited for forming a paper sheet and is hence very well suited for the paper side, it is usually not suited very well for the machine side. If a paper machine screen is provided with a plain weave paper side, it can therefore be advisable to provide for a second fiber layer underneath the plain weave, forming the machine side of the screen, which gives the screen sufficient stability and wearing potential.
In this respect, the connection of the two layers (i.e. of the upper fabric forming the paper side and the lower fabric forming the machine side) is a particular challenge, amongst others due to the fact that the plain weave favorable for the paper side involves particularly unfavorable preconditions for such a layer connection.
The state of the art describes different approaches for connecting two screen fabric layers, one approach of which describes the use of additional, separate binder threads (not forming part of/not fitting into the structure and the weave pattern, respectively) extending in a longitudinal direction and/or a transverse direction. According to this approach, two finished and completed fabric layers are connected to each other by separate/additional binder threads, which binder threads do not contribute to/are not required for forming the respective fabric layer weave. Both fabric layers consist of longitudinal threads and transverse threads which extend exclusively in the respective fabric layer and thereby completely generate the respective fabric layer pattern or fabric layer weave. This approach is, for example, described in CA 1 115 177 A1, where separate binder weft threads are used which bind with warp threads of the upper fabric and warp threads of the lower fabric, and in DE 39 28 484 A1 in which separate warp threads are used as binder threads. Other examples can be found in DE 42 29 828 A1, WO 93/00472, and EP 0 136 284 A2. The separate binder threads are usually configured to be thinner than the threads forming the respective fabric layer (cf., for example, CA 1 115 177 A1), as the binder threads must be incorporated in the fabric structure in addition to the fabric forming threads. In this respect, little space is provided for such separate binder threads especially in a plain weave. Otherwise, the binder threads would interfere with the originally homogeneous structure of the weave, so that imperfections which cause markings in the paper would be produced especially in the plain weave provided on the paper side. However, practice has shown that the thin binder threads wear out and break rather fast in particular in paper machines which process a high amount of abrasive fillers or the construction of which puts heavy bending loads on the screens in the machine direction, so that the two fabric layers are first displaced and then separated as a result thereof. It goes without saying that it is impossible to make high quality paper by means of a fabric/screen changed in such a manner.
An alternative is the use of structural threads (belonging to/contributing to the structure and to the weave pattern, respectively, of at least one fabric layer) for connecting the layers. In this respect, the threads used for connecting the layers (“binding threads”) on the one hand serve for connecting the layers, for which purpose they alternate between the layers, and, on the other hand, participate in forming the upper fabric and/or the lower fabric (especially the respective recurring characteristic weave or overlapping pattern). Different structural threads may be used as connecting threads, for example transverse threads (or alternatively longitudinal threads) which participate in structurally forming the upper fabric, the different structural threads bringing about different screen properties.
In addition/in this respect, it is known to use two transverse threads arranged adjacently in a longitudinal direction, which interact as a so-called functional transverse thread pair. In this respect, both transverse threads of a functional pair together alternately form a virtually uninterrupted transverse thread on the paper side, which fits in the weave pattern of the paper side and may, for example, form part of a paper side plain weave. Those thread portions of the functional pair which are currently not required for forming the virtually uninterrupted transverse thread on the paper side extend in the interior of the fabric and can be used for binding the lower fabric to the upper fabric. In this respect, the thread portion binding the lower fabric can, for example, complete the lower fabric or its weave at the same time. For example, one or both transverse threads of a functional pair may alternately extend in the upper fabric and the lower fabric. An upper transverse thread may, for example, be provided between two functional transverse thread pairs, which completes exclusively the plain weave (i.e. which extends only in the upper fabric), but has no binding function. Exemplary embodiments of this approach can be found, for example, in EP 0 097 966 A2, EP 794 283 A1, WO 99/06630 A1, WO 99/06632 A1, and WO 02/14601 A1.
Alternatively, the layers may be connected by so-called functional longitudinal thread pairs. EP 0 069 101 and EP 093 096 are pointed out as examples in this regard, showing a layer connection through functional longitudinal thread pairs.
With respect to the fabrics connected through functional transverse thread pairs, reference is in addition made to the following patent literature.
EP 1 021 616 B1 by Kevin J. Ward shows a fabric in which the paper side is exclusively formed by functional transverse thread pairs in the transverse direction, i.e. there are no pure upper transverse threads (see FIG. 1a of said patent). The paper side of such a screen has a comparatively high tendency to marking, as there is no support of the upper longitudinal threads and no stabilization of the upper fabric, respectively, by pure upper transverse threads. The paper side realized by a plain weave becomes irregular due to the missing upper transverse threads and the great number of changeover positions. Moreover, a great amount of transverse threads is used.
In patent EP 1 311 723 B1 by Heinz Odenthal a fabric is shown in which the paper side is exclusively formed by functional transverse thread pairs in the transverse direction, wherein only in every second pair one of the two transverse threads is formed as a binding thread which immerges in the lower fabric layer (see FIG. 3 of said patent). In addition to an increased tendency to marking, this fabric thus has a comparatively small number of binding positions (with a high number of introduced transverse threads).
EP 1 754 820 A1 by Johann Boeck shows a fabric in which two pure upper transverse threads and one functional transverse thread pair are arranged alternately one after another in a longitudinal direction on the paper side (see FIG. 1 of said publication). Due to the comparatively small number of functional transverse thread pairs, the number of binding positions is relatively low.
Fabrics in which one upper transverse thread and one functional transverse thread pair are arranged one after another in a longitudinal direction on the paper side are, for example, described in EP 1 000 197 B1 by Kevin J. Ward, EP 1 158 089 B1 by Kevin J. Ward, EP 1 158 090 B1 by Kevin J. Ward, WO 2010/041123 A2 by Clara Rosetti, EP 0 794 283 B1 by Dale B. Johnson et al., U.S. Pat. No. 5,826,627 by Ronald H. Seabrook, WO 2004/111333 A2 by Steward L. Hay and WO 2005/014926 A1 by Stewart L. Hay. Also these fabrics have a relatively high tendency to marking or an irregular paper side, as one upper longitudinal thread in two is supported exclusively by functional transverse thread pairs which form changeover positions. In other words, all pure upper wefts and upper transverse threads, respectively, rest on the same warp threads and upper longitudinal threads, respectively (especially on one warp thread in two).
Other examples show structures with a longitudinal thread ratio or warp thread ratio of upper fabric to lower fabric of unequal to 1 (in this respect, a ratio of 1:1 can be realized particularly easily). Solutions of this type are published in the patents EP 1 849 912 B1 by Kevin J. Ward, in which a warp ratio of upper fabric to lower fabric of 3 to 2 is described and in which, in addition, one functional transverse thread pair and one upper transverse thread are arranged one after another in a longitudinal direction on the paper side, and U.S. Pat. No. 7,487,805 B2 by Christine Baratte, Steward Hay and Kevin J. Ward, in which a weft-bound fabric having a warp ratio of upper warp to lower warp of less than 1 is described and in which, in addition, one functional transverse thread pair and two upper transverse threads are arranged one after another in a longitudinal direction on the paper side.
In both patents EP 1 002 892 B1 by Ralf Kaldenhoff and EP 2 205 791 B1 by Petra Hack-Ueberall and Arved Westerkamp, the simultaneous use of warp thread pairs and weft thread pairs is proposed as a solution for a stable layer connection.
The present invention now describes a paper machine screen which forms part of the group of screens/fabrics, the layers of which are connected through functional transverse thread pairs. Within this group, the screen according to the invention forms part of the subgroup where the two transverse threads of a respective functional pair alternately complete the upper weave (i.e. provide a structural, virtually uninterrupted transverse thread for the upper fabric layer) and only bind the lower weave or layer to the upper layer, i.e. do not contribute to forming the lower weave pattern. In addition, the screen according to the invention forms part of the subgroup in which both transverse threads of a respective transverse thread pair are formed as binding transverse threads, so that a great number of bindings can be achieved.
It is an object of the present invention to provide a sheet forming screen made of a multi-layer fabric, which meets the requirements described above at least in part, e.g. completely, (i.e., for example, comprises or combines a high fiber support, a low tendency to marking, an appropriate mechanical stability and a stable layer connection), and which, in addition, can be realized easily.